velociraptor: fix label actions (stray quote after FROM scope()) (fixes #459)

[chiefghost47]

Problem

add_client_label, get_client_label and remove_client_label never do anything (#459). Each query
ends with a stray single quote after FROM scope():

query = "SELECT label(client_id='" + client_id + "', labels=['" + label  +"'], op='set') FROM scope()'"

Velociraptor does not run the statement: the actions return [], and no label is set, checked or
removed.

Fix

Remove the trailing ' from the three label queries. Nothing else changes.

Testing

Against Velociraptor 0.77.3, calling the app's own methods in an image built with the repo's
Dockerfile:

• Before: add_client_label returns [] and the label is not on the client.
• After: add_client_label sets the label ({"ClientId": ..., "Labels": [...], "Op": "set"}),
  get_client_label returns the check result, and remove_client_label removes it. Each step was
  confirmed by reading the client's labels afterwards.

The test image also carried #457 (org_id), so the calls could be run in a non-root org. This change
does not depend on it.

Fixes #459